The first electronic mail systems used file transfer protocols where the recipient's address was in the first line of the message sent as a file. More elaborate electronic mail systems have been defined and described in the RFC (Request For Comments) documents. Mail transmission protocols have been standardized such as the Simple Mail Transfer Protocol (SMTP), RFC 2821 and the Internet Message Format RFC 2822. According to the SMTP model, the sender of a message provides in addition to the normal text, additional information which are sent in the header of the message. The author of the mail indicates:                a sender address (‘From:’ field in the mail header),        a recipient address which may be the address of final recipient (‘To:’ field),        the address of the people in copy (‘Cc:’ field), and        the address of people in ‘Blind Carbon Copy’ (‘Bcc:’ field).        
The “Bcc:” field comprises the address of the recipients of the message whose address must not be revealed to the other recipients of the message. There are three ways in which the “Bcc:” field is used (refer to RFC2822 Internet Message Format):                In the first case, when a message comprising a “Bcc:” field is prepared to be sent, the “Bcc:” line is removed even though all of the recipients (including those specified in the “Bcc:” field) are sent a copy of the message.        In the second case, a copy of the message with the “Bcc:” line removed as above, is sent to the recipients specified in the “To:” and “Cc:” lines. However, the recipients on the “Bcc:” line get a separate copy of the message with a “Bcc:” line (When there are multiple recipient addresses in the “Bcc:” field, in some implementations, the “Bcc:” line comprises only the address of the recipient).        Finally, since a “Bcc:” field may contain no address, a “Bcc:” field can be sent without any addresses indicating to the recipients that blind copies were sent to someone.        
The Simple Mail Transfer Protocol (SMTP), by introducing the recipient fields “To:”, “Cc:” and “Bcc:” takes into account the need of performing two operations:                sending a mail to one or several primary recipients (“To:” field) and officially informing other recipients of this sending (the “Cc:” field), or        sending a mail to one or several primary recipients (“To:” field), officially informing other recipients of the sending (the “Cc:” field), and independently and unofficially informing other recipients of the sending (the “Bcc:” field).        
SMTP can send mails to these three kinds or recipients without duplicating the sending operations. SMTP proposes a function for simplifying the management of the addresses, mainly based on the concept of Directories and Distribution Lists. Directories can be based either on a general shared Directory or on Local Address Books. Directories comprise distribution lists to facilitate the sending to multiple recipients. According to SMTP, a mailbox is a virtual storage entity which receives mail for a particular recipient. A distribution list allows a sender to name a group of recipients without actually providing an individual mailbox address for each recipient. When a sender creates a message, he/she puts the name of the distribution list. In case of local distribution list, the mail application operating on his/her workstation automatically expands said distribution list by replacing the distribution list name by the address of each member of the distribution list. Otherwise (If the distribution list is not local), the distribution list name is inserted in the list of recipients. The distribution list will be expanded by the MTA owner of this distribution list. So the Header of the message may comprise both a list of mail boxes address and distribution list names. The sender has the possibility to request (or not) a mail acknowledgment from the recipients.
Message senders may want to request a mail acknowledgment only from a subset of recipients. For example the sender wants to know whether or not the main recipients (“To:” field) have read the mail and does not want to be polluted by acknowledgments returned by people in copy (“Cc:” field) or people in blind carbon copy (“Bcc:” field). With conventional mail systems, the process to meet this requirement is tedious and prone to error. The mail originator must send:                a first mail to the recipients from whom the mail originator wants to receive an acknowledgment (said first mail including a request for an acknowledgment).        a second mail, identical to the first one, to another list of recipients from whom the mail originator does not want to receive any acknowledgment.        
This process is clearly not user friendly and has a major drawback: the recipients listed in a mail for which an acknowledgment has been requested, are not aware of all the recipients of this mail. In particular they are not aware of the recipients for whom no acknowledgment has been requested (and vice versa). Consequently a reply to all recipients sent by a recipient of an acknowledged mail, will not be received by all the recipients, and in particular will not be received by the recipients for whom no acknowledgment has been requested.
Thus, there is a need in current mail systems, to facilitate the operation of requesting an acknowledgment when this acknowledgment is requested only for a part of the recipients.